*{
   margin: 0;  /*外间距*/
   padding: 0; /*内间距*/
} 
 .header{
    width: 100%;
    height: 600px;
    background-color: rgb(244, 235, 237);
    min-width: 400pxpx;
 }
 /* 块级元素：占据一整行 */
 .div1{
    width: 300px;
    height:120px;
    background-color: lightseagreen;    
    float: left; /*会脱离原来的位置*/
 }
 .div2{
    width: 400px;
    height: 120px;
    background-color: lawngreen;
    float: left;
 }
 .div3{
    width: 300px;
    height: 130px;
    background-color: yellow;
    float: left;
 }
